[volt-nuts] HP 3456A Setup

[Charles P. Steinmetz]

>As far as zeroing the meter with its leads shorted, using "Math 
>Null" (aka Tare) is the only way to zero the 3456A's small offset 
>that is left over after "auto zero".

>When using the Null command with the 3456A's  slow update rate, high 
>resolution, and filter off, you can measure voltages down to 0.1 uv 
>if you're real careful.

My point was that doing this most likely does nothing to reduce 
systematic errors -- rather, it just adjusts readings by the 
particular random noise sample it takes when you activate the Null function.

Certainly, you can READ a 3456A to 0.1 uV on its lowest range, but so 
what?  There is a big difference between resolution and 
accuracy.  You have to be very careful that those glowing red digits 
don't lull you into thinking you have measured something with an 
accuracy of 0.1 uV just because the last digit reads tenths of a 
uV.  In fact, the ACCURACY (uncertainty) of the reading is much, much 
poorer than 0.1 uV -- in this case, well over +/- 2 uV.

If you look at the 3456A specs, you will see that the best specified 
accuracy on the lowest range is +/- 22 ppm +/- 24 counts -- and 
that's only for 24 hours.  (At 90 days it is +/- 34 ppm +/- 24 
counts.)  On the lowest range, 24 counts is 2.4 uV.  So even without 
any scale errors, you can only expect useful accuracy of +/- 2.4 
uV.  The scale error can add another 2 or 3 uV to this, depending on 
the voltage being measured.  So, you can read a 3456A down to 0.1 uV, 
but you can't say that you MEASURED a voltage down to 0.1 uV with it.

We need to be very careful not to get sucked into the resolution 
trap.  It is accuracy that volt nuts are interested in.

Best regards,

Charles